Lecture 4: Amphibian development

Question 1

What are organisers?

Answer 1

• Organisers are parts of the body which direct parts of the body to become certain tissues.
• It can self-differentiate dorsal mesoderm.
• It can dorsalise the surrounding mesoderm into paraxial mesoderm instead of ventral mesoderm. ***
The cells of the organiser contribute to four cell types:
• Pharyngyal endoderm (fore and mid brain).
• Head mesoderm (hindbrain and trunk).
• Notochord.
• Tip of the tail.

Question 2

How does the fertilised egg develop?

Answer 2

• The ventral and dorsal sides are already decided.
• Beta-catenin is degraded in one side but not on the other.
• This leads to differential activation of genes.
• An embryo (16-cells) was injected with goosecoid mRNA at the ventral vegetal pole. This gave a second axis. There were two complete sets of head structures precursors. This did not happen when embryos were given a DNA-binding homeodomain mutant.